Sheep pregnancy in high-altitude environments frequently involves hypoxia and oxidative stress and causes intrauterine growth retardation. The adverse effects of altitude on fetal growth can be prevented by the administration of antioxidant vitamins, but the mechanisms responsible are not well known. The maintenance of a viable pregnancy depends largely on adequate placental steroidogenesis, especially in the last two-thirds of pregnancy. Thus, in the present study we evaluated the effect of antioxidant vitamins (C and E) on plasma concentrations of progesterone and 17β-oestradiol during the last two-thirds of high-altitude pregnancies in ewes both native and naïve to the high-altitude environment. In addition, pregnancy outcomes were evaluated by determining the bodyweight of newborn lambs. Sex steroid patterns differed between ewes with and without vitamin supplementation. Concentrations of plasma progesterone and 17β-oestradiol were significantly higher in the supplemented groups from approximately 40 days before parturition until near term. Newborn weights were significantly lower in animals not adapted to the higher altitude, and vitamin supplementation prevented this decrease. In conclusion, the administration of antioxidant vitamins in the present study enhanced placental steroidogenesis, thus favouring fetal development in pregnancies developing at high altitudes.